Thanks you for everyone's time and effort on this, especially Talacharn, I can't thank you enough.
Although there is still conflicting information. My current thoughts on this are that the Plasbach's on the maps are unfortunately red herrings. The Plasbach that William Treharne lived at and then his daughter Ann, who married a David Owens I don't think is detailed on maps. In 1851 it is a small property (7 acres), which had expanded to 20 acres by 1861. Before 1851 it may have been even smaller, a truly small mansion!
The above is based on looking again at Plasbach in the 1851 and 1861 census and in particular the 1861 census where I have tried to follow the route the enumerator may have taken. Plasbach definitively is in the same area as Van, Llwynyfynnon, Dany yr Allt, Van, Blaen-Meilion etc. and I think Plasbach may have originally just been part of the Quarry "complex".
Also of significance are that South East/South of Van/Fan are Minke (Meinciau) and Ffos Wilkin where William's sons John and Griffith Treharne lived.
Trying to follow the path of the enumerator in 1861 on these maps. The sequence doesn’t seen entirely logical but the general path as seen on the below maps is:
https://maps.nls.uk/view/10217973817 Halfway (Halfway House) – Public House then left to
18 Pantgwenny (Pant-gwinau) (Farm 0f 79 acres), then south to
19 Dan yr Allt (Farm of 40 Acres), the NE to
20 Bancymillon (Blaen-million) (Farm of 35 Acres), then SE to
21 Pantymillon (Pant-y-meillion) (Farm of 40 acres), then SE
22-29 via various properties not on map housing Ag. Labourers, Shoemakers etc.
30 Van (Fan) where Richard Edmund a Mason lived, then S to
31 Llwynyffynnon (farm of 29 acres)
Then via various properties not on the map, housing Lime Burners, Shoemakers, Ag. Labourers and a Blaksmith’s widow, presumably all resident within the Quarry “complex”, including
38 Plasebach where Ann Owen (nee Treharne, William’s daughter) and her husband and family were living (Farmer of 20 Acres), Then SE to
https://maps.nls.uk/view/10217975643 Blanevan (Blaen-y-fan) (Farm of 70 Acres), then SE to
46-48 Minke (Meinciau)
In 1851 the enumerator seems to have travelled in the opposite direction